1. Field of the Invention:
The present invention relates to a logic circuit, and more particularly to an ECL (Emitter Coupled Logic) circuit suitable for being formed in a semiconductor substrate.
2. Description of the Prior Art:
The ECL circuit in the prior art is formed of a current switch section receiving an input logic signal and an emitter follower output section. The current switch section includes a reference transistor, at least one switching transistor having an emitter connected to the emitter of the reference transistor, and a current source transistor connected to the commonly connected emitters of the reference and switching transistors. The base(s) of the switching transistor(s) receives the input logic signal(s) and the base of the reference transistor is held at a reference voltage generated from a first constant voltage source. The base of the current source transistor is held at another constant voltage generated from a second constant voltage source. By the suitable circuit arrangement of the second constant voltage source, the constant voltage can be made free from temperature variation. The small variation of the reference voltage does not affect the operating current. Thus, the operating current is not changed by the change in the reference and constant voltages.
The current switch section of the ECL circuit further includes a first load resistor connected to the collector of the reference transistor, a second load resistor connected in common to the collector(s) of the switching transistor(s), and an emitter resistor of the current source transistor. The ECL circuit further comprises an emitter-follower transistor having a base connected to an output node between the second load and the switching transistor(s) and an emitter resistor thereof, in the emitter follower output section. The resistances of those four resistors employed in the ECL circuit have temperature dependency which is not compensated by circuit arrangement. For example, the diffusion resistor formed by an impurity diffusion into a semiconductor substrate has a positive temperatre coefficient. Therefore, if the operating temperature changes, the current flowing through the ECL circuit changes. More specifically, if the resistors have a positive temperature dependency, the operating current decreases with an increase in temperature.
Generally, transistor circuit has such tendency that the delay time of the output signal from the input signal becomes large as the operating current decreases. Therefore, the ECL circuit has a drawback that the delay time depends on the operating temperature. Furthermore, since the operating temperature changes by ambient temperature and operating condition, the time delay is unstable and unpredictable. Due to the instability of operating current, power consumption is also unstable and unpredictable. These facts make the system design difficult.